protocol:
{
  "driver": "rbd",
  "location": {
    "pool": "asdf",
    "image": "i.qcow2",
    "server": [
      {
        "host": "example.com",
        "port": "1234"
      },
      {
        "host": "alternate.example.com",
        "port": "4321"
      }
    ]
  },
  "size": 4294967296
}

format:
{
  "driver": "qcow2",
  "file": "0123456789ABCDEF0123456789ABCDE",
  "size": 8589934590
}